This invention pertains generally to precursors and deposition methods for thin film nanoporous aerogels on semiconductor substrates, including deposition methods suited to aerogel thin film fabrication of nanoporous dielectrics.
Aerogels are porous silica materials which can be used for a variety of purposes including as films (e.g. as electrical insulators on semiconductor devices or as optical coatings) or in bulk (e.g. as thermal insulators). For ease of discussion, the examples herein will be mainly of usage as electrical insulators on semiconductor devices.
Semiconductors are widely used in integrated circuits for electronic devices such as computers and televisions. Semiconductor and electronics manufacturers, as well as end users, desire integrated circuits which can accomplish more in less time in a smaller package while consuming less power. However, many of these desires are in opposition to each other. For instance, simply shrinking the feature size on a given circuit from 0.5 microns to 0.25 microns can increase energy use and heat generation by 30%. Miniaturization also generally results in increased capacitive coupling, or crosstalk, between conductors which carry signals across the chip. This effect both limits achievable speed and degrades the noise margin used to insure proper device operation. One way to reduce energy use/heat generation and crosstalk effects is to decrease the dielectric constant of the insulator, or dielectric, which separates conductors. U.S. Pat. No. 5,470,802, issued to Gnade et al., provides background on several of these schemes.
A class of materials, nanoporous dielectrics, includes some of the most promising new materials for semiconductor fabrication. These dielectric materials contain a solid structure, for example of silica, which is permeated with an interconnected network of pores having diameters typically on the order of a few nanometers. These materials may be formed with extremely high porosities, with corresponding dielectric constants typically less than half the dielectric constant of dense silica. And yet despite their high porosity, it has been found that nanoporous dielectrics may be fabricated which have high strength and excellent compatibility with most existing semiconductor fabrication processes. Thus nanoporous dielectrics offer a viable low-dielectric constant replacement for common semiconductor dielectrics such as dense silica.
The preferred method for forming nanoporous dielectrics is through the use of sol-gel techniques. The word sol-gel does not describe a product but a reaction mechanism whereby a sol, which is a colloidal suspension of solid particles in a liquid, transforms into a gel due to growth and interconnection of the solid particles. One theory is that through continued reactions within the sol, one or more molecules in the sol may eventually reach macroscopic dimensions so that it/they form a solid network which extends substantially throughout the sol. At this point (called the gel point), the substance is said to be a gel. By this definition, a gel is a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. As the skeleton is porous, the term xe2x80x9cgelxe2x80x9d as used herein means an open-pored solid structure enclosing a pore fluid.
One method of forming a sol is through hydrolysis and condensation reactions, which can cause a multifunctional monomer in a solution to polymerize into relatively large, highly branched particles. Many monomers suitable for such polymerization are metal alkoxides. For example, a tetraethoxysilane (TEOS) monomer may be partially hydrolyzed in water by the reaction
Si(OEt)4+H2Oxe2x86x92HOxe2x80x94Si(OEt)3+EtOH 
Reaction conditions may be controlled such that, on the average, each monomer undergoes a desired number of hydrolysis reactions to partially or fully hydrolyze the monomer. TEOS which has been fully hydrolyzed becomes Si(OH)4. Once a molecule has been at least partially hydrolyzed, two molecules can then link together in a condensation reaction, such as
(OEt)3Sixe2x80x94OH+HOxe2x80x94Si(OH)3xe2x86x92(OEt)3Sixe2x80x94Oxe2x80x94Si(OH)3+H2O 
or
(OEt)3Sixe2x80x94OEt+HOxe2x80x94Si(OEt)3xe2x86x92(OEt)3Sixe2x80x94Oxe2x80x94Si(OEt)3+EtOH 
to form an oligomer and liberate a molecule of water or ethanol. The Sixe2x80x94Oxe2x80x94Si configuration in the oligomer formed by these reactions has three sites available at each end for further hydrolysis and condensation. Thus, additional monomers or oligomers can be added to this molecule in a somewhat random fashion to create a highly branched polymeric molecule from literally thousands of monomers. An oligomerized metal alkoxide, as defined herein, comprises molecules formed from at least two alkoxide monomers, but does not comprise a gel.
Sol-gel reactions form the basis for xerogel and aerogel film deposition. In a typical thin film xerogel process, an ungelled precursor sol may be applied (e.g., spray coated, dip-coated, or spin-coated) to a substrate to form a thin film on the order of several microns or less in thickness, gelled, and dried to form a dense film. The precursor sol often comprises a stock solution and a solvent, and possibly also a gelation catalyst that modifies the pH of the precursor sol in order to speed gelation. During and after coating, the volatile components in the sol thin film are usually allowed to rapidly evaporate. Thus, the deposition, gelation, and drying phases may take place simultaneously (at least to some degree) as the film collapses rapidly to a dense film. In contrast, an aerogel process differs from a xerogel process largely by avoiding pore collapse during drying of the wet gel. Some methods for avoiding pore collapse include wet gel treatment with condensation-inhibiting modifying agents (as described in Gnade ""802) and supercritical pore fluid extraction.
Between aerogels and xerogels, aerogels are the preferable of the two dried gel materials for semiconductor thin film nanoporous dielectric applications. Typical thin film xerogel methods produce films having limited porosity (up to 60% with large pore sizes, but generally substantially less than 50% with pore sizes useful in submicron semiconductor fabrication). While some prior art xerogels have porosities greater than 50%; these prior art xerogels had substantially larger pore sizes (typically above 100 nm). These large pore size gels have significantly less mechanical strength. Additionally, their large size makes them unsuitable for filling small (typically less than 1 mm, and potentially less than 100 nm) patterned gaps on a microcircuit and limits their optical film uses to only the longer wavelengths. A nanoporous aerogel thin film, on the other hand, may be formed with almost any desired porosity coupled with a very fine pore size. Generally, as used herein, nanoporous materials have average pore sizes less than about 25 nm across, but preferably less than 20 nm (and more preferably less than 10 nanometers and still more preferably less than 5 nanometers). In many formulations using this method, the typical nanoporous materials for semiconductor applications may have average pore sizes at least 1 nm across, but more often at least 3 nm. The nanoporous inorganic dielectrics include the nanoporous metal oxides, particularly nanoporous silica.
In many nanoporous thin film applications, such as aerogels and xerogels used as optical films or in microelectronics, the precise control of film thickness and aerogel density are desirable. Several important properties of the film are related to the aerogel density, including mechanical strength, pore size and dielectric constant. It has now been found that both aerogel density and film thickness are related to the viscosity of the sol at the time it is applied to a substrate. This presents a problem which was heretofore unrecognized. This problem is that with conventional precursor sols and deposition methods, it is extremely difficult to control both aerogel density and film thickness independently and accurately.
Nanoporous dielectric thin films may be deposited on patterned wafers, often over a level of patterned conductors. It has now been recognized that sol deposition should be completed prior to the onset of gelation to insure that gaps between such conductors remain adequately filled and that the surface of the gel remains substantially planar. To this end, it is also desirable that no significant evaporation of pore fluid occur after gelation, such as during aging. Unfortunately, it is also desirable that the gel point be reachable as soon after deposition as possible to simplify processing, and one method for speeding gelation of thin films is to allow evaporation to occur. It is recognized herein that a suitable precursor sol for aerogel deposition should allow control of film thickness, aerogel density, gap fill and planarity, and be relatively stable prior to deposition, and yet gel relatively soon after deposition and age without substantial evaporation.
A method has now been found which allows controlled deposition of aerogel thin films from a multi-solvent precursor sol. In this method, sol viscosity and film thickness may be controlled relatively independently. This allows film thickness to be rapidly changed from a first known value to a second known value which can be set by solvent ratios and spin conditions, thus keeping film thickness largely independent of aerogel density and allowing rapid gelation. However, at the same time, the solid:liquid ratio present in the film at drying (and therefore the aerogel density) can be accurately determined in the precursor sol prior to deposition, independent of spin conditions and film thickness.
Even with this novel separation of the deposition problem into viscosity control and density control subproblems, our experience has been that thin film sol-gel techniques for forming xerogels and aerogels generally require some method, such as atmospheric control, to limit evaporation before drying, such as after gelation and during aging. In principle, this evaporation rate control can be accomplished by controlling the solvent vapor concentration above the wafer. However, our experience has shown that the solvent evaporation rate is very sensitive to small changes in the vapor concentration and temperature. In an effort to better understand this process, we have modeled the isothermal vaporization of several solvents from a wafer as a function of percent saturation. The ambient temperature evaporation rates for some of these solvents are given in FIG. 1. For evaporation to not be a processing problem, the product of the evaporation rate and processing time (preferably on the order of minutes) should be significantly less than the film thickness. This suggests that for solvents such as ethanol, the atmosphere above the wafer would have to be maintained at over 99% saturation. However, there can be problems associated with allowing the atmosphere to reach saturation or supersaturation. Some of these problems are related to condensation of an atmospheric constituent upon the thin film. Condensation on either the gelled or ungelled thin film has been found to cause defects in an insufficiently aged film. Thus, it is generally desirable to control the atmosphere such that no constituent is saturated.
Rather than using a high volatility solvent and precisely controlling the solvent atmosphere, we have discovered that a better solution is to use a low volatility solvent with less atmospheric control. Upon investigating this premise, we have discovered that glycerol makes an excellent solvent.
The use of glycerol allows a loosening (as compared to prior art solvents) of the required atmospheric control during deposition, gelation, and/or aging. This is because, that even though saturation should still preferably be avoided, the atmospheric solvent concentration can be lowered without excessive evaporation. FIG. 2 shows how the evaporation rate of glycerol varies with temperature and atmospheric solvent concentration. It has been our experience that, with glycerol, acceptable gels can be formed by depositing, gelling and aging in an uncontrolled or a substantially uncontrolled atmosphere.
In the production of nanoporous dielectrics it is preferable to subject the wet gel thin film to a process known as aging. Hydrolysis and condensation reactions do not stop at the gel point, but continue to restructure, or age, the gel until the reactions are purposely halted. It is believed that during aging, preferential dissolution and redeposition of portions of the solid structure produce beneficial results, including higher strength, greater uniformity of pore size, and a greater ability to resist pore collapse during drying. Unfortunately, we have now found that conventional aging techniques used for bulk gels are poorly suited for aging thin films in semiconductor processing, partly because they generally require liquid immersion of the substrate and partly because they require days or even weeks to complete. One aspect of this invention includes a vapor phase aging technique that avoids liquid immersion or premature drying of the wet gel thin film and that, surprisingly, can age such a thin film in a matter of minutes.
Again, aerogels are nanoporous materials which can be used for a variety of purposes including as films or in bulk. It should be noted, however, the problems incurred in film fabrication processing is so different from bulk processing problems, that, for practical purposes, film processing is not analogous to bulk processing.
Generally, we have now found that aging in a saturated atmosphere avoids the difficulties encountered with liquid immersion aging. Furthermore, this aspect of the invention provides several approaches for aging wet gels at increased temperatures. These methods may be used even when the wet gel originally contains low boiling point pore liquids. However, they work better with low volatility solvents. Finally, this aspect of the invention provides for adding an optional vapor phase aging catalyst to the aging atmosphere to speed aging.
Aging a wet gel in thin film form is difficult, as the film contains an extremely small amount of pore fluid that should be held fairly constant for a period of time in order for aging to occur. If pore fluid evaporates from the film before aging has strengthened the network, the film will tend to densify in xerogel fashion. On the other hand, if excess pore fluid condenses from the atmosphere onto the thin film before the network has been strengthened, this may locally disrupt the aging process and cause film defects.
Thus, we now know that some method of pore fluid evaporation rate control during aging is beneficial to aerogel thin film fabrication. In principle, evaporation rate control during aging can be accomplished by actively controlling the pore fluid vapor concentration above the wafer. However, the total amount of pore fluid contained in, for instance, a 1 mm thick 70% porous wet gel deposited on a 150 mm wafer is only about 0.012 mL, an amount that would easily fit in a single 3 mm diameter drop of fluid. Typical thin films used for nanoporous dielectrics on semiconductor wafers are approximately 1000 times thinner. Thus, actively controlling the pore fluid vapor concentration (by adding or removing solvent to the atmosphere) to allow no more than, e.g., 1%, or less, pore fluid evaporation during aging presents a difficult proposition; the surface area of the thin film is high and the allowable tolerance for pore fluid variations is extremely small. In particular, evaporation and condensation control are especially important for rapid aging at elevated temperature, where film production processes have heretofore apparently not been practically possible.
We have overcome the evaporation rate control problem by not attempting to actively control pore fluid vapor concentration above a wafer at all. Instead, the wafer is processed in an extremely low-volume chamber, such that through natural evaporation of a relatively small amount of the pore fluid contained in the wet gel film, the processing atmosphere becomes substantially saturated in pore fluid. Unless the wafer is cooled at some point in a substantially saturated processing atmosphere, this method also naturally avoids problems with condensation, which should generally be avoided, particularly during high temperature processing.
A method for forming a thin film nanoporous dielectric on a semiconductor substrate is disclosed herein. This method comprises the steps of providing a semiconductor substrate and depositing an nanoporous aerogel precursor sol upon the substrate. This aerogel precursor sol comprises a metal-based aerogel precursor reactant and a first solvent comprising glycerol; wherein, the molar ratio of the molecules of glycerol to the metal atoms in the reactant is at least 1:16. The method further comprises allowing the deposited sol to create a gel, wherein the gel comprises a porous solid and a pore fluid; and forming a dry, nanoporous dielectric by removing the pore fluid in a drying atmosphere without substantially collapsing the porous solid. In this method, the pressure of the drying atmosphere during the forming step is less than the critical pressure of the pore fluid, preferably near atmospheric pressure.
Preferably, the aerogel precursor reactant may be selected from the group consisting of metal alkoxides, at least partially hydrolyzed metal alkoxides, particulate metal oxides, and combinations thereof. Preferably, the aerogel precursor reactant comprises silicon. In some embodiments, the aerogel precursor reactant is TEOS. Typically, the molar ratio of the molecules of glycerol to the metal atoms in the reactant is no greater than 12:1, and preferably, the molar ratio of the molecules of glycerol to the metal atoms in the reactant is between 1:2 and 12:1. In some embodiments, the molar ratio of the molecules of glycerol to the metal atoms in the reactant is between 2.5:1 and 12:1. In this method, it is also preferable that the nanoporous dielectric has a porosity greater than 60% and an average pore diameter less than 25 nm. In some embodiments, the aerogel precursor also comprises a second solvent. Preferably, the second solvent has a boiling point lower than glycerol""s. In some embodiments, the second solvent may be ethanol. In some embodiments, the first solvent also comprises a glycol, preferably selected from the group consisting of ethylene glycol, 1,4-butylene glycol, 1,5-pentanediol, and combinations thereof. After aging but before drying, in some embodiments, the aging fluid is replaced by a drying fluid. This allows, e.g., rapid, lower temperature (e.g., room temperature) drying with a fluid that evaporates faster and has a suitably low surface tension. Examples of drying fluids include heptane, ethanol, acetone, 2-ethylbutyl alcohol and some alcohol-water mixtures.
Thus, this invention allows controlled porosity thin film nanoporous aerogels to be deposited, gelled, aged, and dried without atmospheric controls. In another aspect, this invention allows controlled porosity thin film nanoporous aerogels to be deposited, gelled, rapidly aged at an elevated temperature, and dried with only passive atmospheric controls, such as limiting the volume of the aging chamber.